The present invention relates to a sodium-sulfur storage battery in which molten sodium is used as a negative reactant, molten sulfur-sodium polysulfide is used as a positive reactant and a sodium-ion conductive solid electrolyte tube is used as an electrolyte, and more particularly, to a storage battery in which gas tightness is improved at a solder-glass-joint between an alpha alumina ring and the solid electrolyte tube.
The sodium-sulfur storage battery is a secondary battery in which both reactants are separated by the sodium-ion conductive solid electrolyte tube of, e.g., beta alumina, beta-two-dash alumina or nasicon provided with a bottom and is operated at a high temperature of about 300.degree.-400.degree. C. As shown in FIG. 1, a solid electrolyte tube 2 is inserted into an annular hollow provided in an alpha alumina ring 1, and is heat-treated to be joined together by solder glass 3 filled in a space of 0.1-0.2 mm therebetween. A solder glass surface 4 largely swells from the alpha alumina ring 1 before the joint, and upon the heat-treatment, a solder glass surface 5 retracts toward the bottom of the alpha alumina ring 1 and fixes the solid electrolyte tube 2 and the alpha alumina ring 1 together. However, through heat cycles (between ordinary temperature and 400.degree. C., temperature rising rate: 200.degree. C./H, 400.degree. C.-maintaining time: 30 minutes) during the operation of the battery, cracks 6 are generated initially on the surface, as shown in FIG. 2, and gradually extend to the interior of the solder glass 3 after the two-five cycles, and finally, penetrating cracks 7 are formed. The penetrating cracks 7 break the gastightness with respect to the atmosphere, and thus, the negative reactant, i.e., sodium, in the solid electrolyte tube is oxidized, which reduces an operation capacity of the battery.
Accordingly, it is an object of the invention to provide a sodium-sulfur storage battery, overcoming the above-noted disadvantages.
The present invention is described by way of example, with reference to the accompanying drawings.